We consider the problem of assigning n_1 targets to be illuminated by n_2 radars over n_3 time slots with n_1<=n_2<=n_3, whereby no two radars illuminate the same target in the same time slot, as many targets are illuminated each time as possible, and all radars illuminate all targets over the span of all time slots. The problem is formulated as a planar three-index assignment problem. A branch-and-bound algorithm as well as an approximate Lagrangian relaxation algorithm are presented. The Lagrangian relaxation algorithm can efficiently approximate solutions to very large optimization problems.